The field of the present invention relates generally to audio amplifier systems, and more particularly to such systems that purposely distort the amplified audio signals in a manner to enhance the sound produced to accommodate a user""s sound preference.
The timbre of a musical note is recognizable by the harmonic spectrum associated with the fundamental tone. When a sine wave is applied to an audio device under test (DUT) odd and even order harmonics generated indicate the presence of harmonic distortion.
For audiophile applications, minimization of distortion has become so effective that digital sources (e.g. compact discs, or CD""s) are often characterized as being xe2x80x9ccoldxe2x80x9d or xe2x80x9csterile.xe2x80x9d Judicious amounts of distortion are necessary to xe2x80x9cwarm upxe2x80x9d the signal; the resurgence in popularity of electron vacuum tube circuits in the high-end audiophile market reflects this awareness. Both tube and solid state circuits can be designed to maximize or minimize distortion; tube circuits are inherently more distorted.
The amplification system translates an electrical signal modulated by audio signals to audio or sound waves through a loudspeaker. Speaker design variables can be adjusted to optimize efficiency and low levels of distortion, or early break-up and higher levels of distortion for a given signal amplitude. A speaker cone set in motion by an audio drive signal will generate harmonics associated with the corresponding fundamental frequency. It is generally recognized that even-order harmonics are more musical whereas odd-order harmonics are harsher. The xe2x80x9cwarmxe2x80x9d tone produced by speakers driven to distortion contain subtle harmonics that are recognizable by either a listener or detected by a microphone placed at a distance from the speaker, provided that sound pressure levels are sufficient to result in air movement recognized by the human""s auditory conduction system or the mechanical response parameters of the microphone. This has been long recognized in the recording industry, where very sensitive (ribbon) microphones are used to pick up the signal of low wattage single ended tube amplifiers and less sensitive microphones can be used to pick up the higher Db (decibel) levels from more powerful push-pull tube or solid state amplifiers.
To the listener, the timbre of a note is a combination of the associated fundamental frequency and all associated harmonics thereof produced along the signal path. To hear the harmonics produced by a speaker, the loudspeaker must be either very inefficient (early breakup with low applied drive signal strength) or driven to high sound pressure levels. There is significant attenuation of the harmonic content of the audio signal when passing from the speaker through air to the listener or the microphone.
For example, an acoustic guitar has recognizable and desirable tonal characteristics at low volume levels, the same does not hold true for the electric guitar, for example, and other electric musical instruments. With respect to the electric instrument, tonal characteristics are affected by materials and methods of construction and various design features. Hollow, semi hollow, and solid body guitars all sound different when amplified. Types of pickups/transducers and their placement and wiring schema will affect tone.
The electric instrument tone is derived partially from the instrument itself, as in an acoustic guitar, but is significantly dependent on the amplification system employed. Recognizable and desirable electric tonal parameters include attack/decay envelope, timbre and resonance of the fundamental note and harmonics. One problem for amplifier designers has been the ability to generate acceptable and desirable tonal characteristics at all commonly encountered volume levels.
Another problematic area for electric guitar amplifier designers has been the recognition that optimum electric guitar tone is developed by amplification systems using tube technology. There have been many attempts at introducing solid state technology, with varying degrees of success. Tube amplification is still the gold standard by which all other systems are judged. There has been a recent proliferation of hybrid designs. Unfortunately, the formerly xe2x80x9clost artxe2x80x9d of tube based circuits will only last as long as tube manufacture continues. Although there is presently a thriving market in used and NOS (new-old-stock tubes), manufacture of new tubes is mainly confined to emerging third world nations that still embrace this technology. The quality of these tubes for musical instrument amplification is an issue of concern, as is their long-term availability as these nations shift technology to solid-state based designs. With respect to the electric guitar, tube amplifiers are still the gold standard by which any new amplifications/signal processing device is judged.
The links in the signal processing chain from electric guitar to amplified audible sound have traditionally included:
ELECTRIC GUITAR;
EFFECTS PROCESSORS;
PREAMPLIFIER/POWER AMPLIFIER; AND
SPEAKER.
A major determinant of desirable electric guitar tone is DISTORTION. This can be introduced at any of the stages noted above. The most desirable tone has traditionally been generated by driving the power amplifier and speaker combination to maximum output.
Common variations on the signal processing chain have included the following arrangements:
ELECTRIC GUITAR passive inductive pickups active pickups contact/ pressure transducers
EFFECTS PROCESSORS solid state effects tube effects/hybrid
AMPLIFIER tube preamplifier tube power amplifier solid state preamplifier solid state power amplifier
Hybrid: tube preamplifier solid state power amplifier solid state preamplifier tube power amplifier
Speaker:
Other approaches have involved amplifying the signal at or about this stage and include speakerless line out (Speaker Emulation), further sound reinforcement thru use of a microphone to secondary amplification systems, (as in miking open speaker for live performance thru a Public Address System or into recording console mixer.) Various forms of speaker isolation boxes are also used, mainly in recording studios, with miking of the enclosed speaker.
Distortion:
A clean (no overdrive) guitar signal has the volume envelope of a plucked string, similar to a piano. Characteristically, the electric guitar sound considered by both players and listeners to be most recognizable as having xe2x80x9cgood tonexe2x80x9d has included a degree of overdrive distortion. This is best described as having some similarities to the waveform of a harmonica or a saxophone although these instruments have characteristic resonance, timbre and volume envelopes which clearly distinguish them from an overdriven electric guitar sound. The electric guitar""s dynamic range is reduced as more overdrive is used. This helps to mask the difference in attack between a plucked string, fretboard tapping of notes, and picking techniques used to generate string harmonics. More overdrive produces a relatively constant volume, from note striking through decay. The sustain of the volume envelope is thereby increased, allowing the player artistic freedom to employ techniques and styles not possible with an amplified clean guitar sound or an acoustic guitar.
Various descriptive terms for degrees of distortion have evolved. xe2x80x9cBrownxe2x80x9d or xe2x80x9cthickxe2x80x9d refers to subtle overdrive at the start of notes which decays into a cleaner sound. xe2x80x9cClippedxe2x80x9d or xe2x80x9coverdrivexe2x80x9d sounds are rich in harmonics and exhibit long sustain. The preamplifier/amplifier designs noted above have been able to produce the various types of distortion at lower output levels with varying degrees of success. Generally, tone is improved as the preamplifier, power amplifier, and speaker are all pushed to their limits, regardless of the nature (i.e. tube or solid state design) of the component parts. Amplifier manufacturers have taken partial advantage of this by providing separate volume controls for the preamplifier and power amplifier stages in an attempt to promote flexibility in tonal shaping.
Whereas it might seem unusual that amplifier design would be directed toward intentionally promoting distortion, the evolution of guitar amplifiers would indicate that this is the case. Of late, electric guitarists have attempted to get overdriven amplifier/speaker tone by many other methods including low-pass filters, compression, inductive loads, vacuum tube preamplifiers used typically as xe2x80x9ctube pedalsxe2x80x9d, and equalization. These are usually introduced early in the signal path, between the instrument and the amplifier. Overall, these have been less than satisfactory and have introduced among other things, significant ergonomic problems. The portable xe2x80x9ccomboxe2x80x9d amplifier (preamplifier/power amplifier/speaker in one easily transportable cabinet) of the 1960""s has evolved into multi-amplifier, multi-cabinet, rack mounts, high wattage output rigs that are often played at exceedingly high sound pressure levels to achieve the desired tone.
An object of the invention is to provide an improved system for enhancing the tone quality of an audio signal amplified through the system and outputted from a loudspeaker.
Another object of the invention is to provide an improved audio signal amplification system for permitting a listener to match the tonal quality to room acoustics in a manner preferred by the listener.
These and other objects of the invention are obtained by amplifying an audio signal via amplifying means for distorting the sound emitted from a loudspeaker connected to the output of an associated amplifier. A piezo film sensor element or transducer is secured to the cone of the first loudspeaker for obtaining an electrical signal corresponding to the sound being emitted by the first loudspeaker. The electrical audio signal obtained from the transducer is then amplified by a second amplifier, preferably having a flat frequency response, for driving a second loudspeaker, whereby the frequency rolloff and volume level of the sound emitted by the second loudspeaker is adjusted to the preference of a listener for matching the sound output from the entire system to the room characteristics and desired sound pressure levels.